The present invention generally relates to the field of bar code readers, and more particularly to a method and apparatus for scanning and decoding a two-dimensional bar code such as PDF417 using a charge-coupled device (CCD) camera or a charge modulation device (CMD) camera.
Bar codes have become broadly accepted as a means for automatically identifying objects. A bar code symbol is a pattern of parallel bars and spaces of various widths that represent data elements or characters. The bars represent strings of binary ones and the spaces represent strings of binary zeros. Generally, the bars and spaces can be no smaller than a specified minimum width which is called a "module" or "unit." The bars and spaces are multiples of this module size or minimum width.
The conventional bar code symbol is "one-dimensional" in that the bars and spaces extend only in a single direction. There has been an increasing need, however, for machine-treadable symbols that contain more information than conventional bar code symbols. One approach for increasing the information in machine-readable symbols is to reduce the height of the bar codes and stack the bar codes one on top of each other to create a "stacked" or "two-dimensional" bar code. One such two-dimensional bar code is PDF417, which was developed by Symbol Technologies, Inc. A complete description of the PDF417 code is contained in U.S. patent application Ser. No. 07/461,881, filed Jan. 5, 1990 and assigned to the same assignee as the present invention, which is hereby incorporated by reference.
Bar code symbols are typically read by optical techniques, such as scanning laser beams, and the resulting electrical signals are then decoded to recover the data encoded in the symbol. When decoding a two-dimensional bar code symbol, however, laser scanners have the disadvantage that the scan lines must be substantially aligned with the rows of the symbol. Although a two-dimensional bar code such as PDF417 allows some deviation, the orientation of the scan lines must still be less than a maximum angle relative to the rows of the symbol.
There are many applications, however, in which it is desirable to be able to read and decode a two-dimensional bar code symbol in any orientation without having to align the reader with rows of the symbol. For example, in an industrial environment, the symbol may be located on an object moving along a conveyor belt where the reader views the symbol from above. Thus, the symbol may be in any orientation relative to the reader.
In addition, it is often desirable to be able to read a code symbol that has defects. For example, handling may cause a corner of a symbol to be torn off, or a stain or scratch may obscure part of a symbol.